Method for crystallizing carboxylic acid

ABSTRACT

The present invention relates to a process for the crystallization of carboxylic acid, more particularly adipic acid, and a process for manufacturing crystalline carboxylic acid, more specifically a process for treating the reaction medium resulting from the direct oxidation of a hydrocarbon into carboxylic acid such as adipic acid, for example. More specifically, the invention relates to the crystallization of adipic acid from an organic solvent, thus removing the organic impurities contained in the said acid and thus optionally facilitating its purification by a crystallization from water.

This application is a national stage application, filed under 35 U.S.C.371, of PCT/FR99/0281, filed Sep. 14, 1999.

The present invention relates to a process for the crystallization ofcarboxylic acid, more particularly adipic acid, and a process formanufacturing crystalline carboxylic acid, more specifically a processfor treating the reaction medium from the direct oxidation of ahydrocarbon into carboxylic acid such as adipic acid, for example.

Adipic acid is one of the two starting materials in the preparation ofpolyamides, such as polyamide 6-6, and of various other polymers. Theapplications of polyamide 6-6 require highly controlled properties interms of both chemical and physicochemical composition. In order toobtain such a polymer, it is necessary to have very high purity at thestage of the monomers, such as adipic acid or dicarboxylic acids.

Adipic acid is also one of the important starting materials in themanufacture of polyurethanes.

Depending on the process for the synthesis of adipic acid, theimpurities it contains are obviously different. However, it is alwaysnecessary to involve a step of purification of the adipic acid produced.

The purification processes generally used are crystallization processesusing water as solvent. Such a process is described, for example, inFrench patent application No. 2,749,299.

However, depending on the synthetic process used and the nature of theproduct to be purified, the known crystallization solvent, namely water,does not allow certain impurities to be removed, such as organicimpurities, for example.

One of the aims of the present invention is to provide a process for thecrystallization of carboxylic acid, and more particularly of adipicacid, in solvents capable of removing the said organic impurities.

To this end, the invention provides a process for the crystallization ofcarboxylic acid, characterized in that the said crystallization orrecrystallization is carried out from an organic solvent or a mixture ofsolvents, at least one of which is an organic solvent, in which thecarboxylic acid has a solubility of less than or equal to 15% by weightat a temperature of 20° C., and in that the said solvent is chosen fromthe families of organic compounds comprising ether, alcohol, ketone,ester, nitrile, amide, sulphoxide or carbonate functions andhalogen-containing, nitro or phosphorus-containing organic solvents.

According to a preferred characteristic of the invention, the organicsolvent is chosen if the solubility of the carboxylic acid in the saidsolvent at a temperature of 100° C., or at its boiling point if this islower than 100° C., is at least 5% higher than that at 20° C. in thesame solvent, preferably 10% higher. Furthermore, the solubility at 100°C. should advantageously be at least 15% by weight.

The term “higher solubility” should be understood as meaning adifference in solubility as an absolute value.

The carboxylic acids which can advantageously be crystallized by theprocess of the invention are preferably dicarboxylic acids such asadipic acid, succinic acid, glutaric acid, terephthalic acid andisophthalic acid. The preferred carboxylic acid of the invention isadipic acid.

As solvents which are suitable for the process of the invention, mentionmay be made, for example, of 1,4-dioxane; diglyme (diethylene glycoldimethyl ether); aliphatic, cycloaliphatic, aromatic or arylaliphaticketones such as acetone, methyl isobutyl ketone, methyl ethyl ketone,methyl isopropyl ketone, methyl phenyl ketone, cyclohexanone,tetrahydrofuran; n-butanol; isopropanol; 3-methoxyethanol; acetonitrile;dimethylformamide; acetamide; dichloromethane; ethyl acetate;1,2-dichloroethane; dimethyl sulphoxide; nitromethane;N-methylpyrrolidone. This list is not exhaustive, the use of otherorganic solvents which satisfy the general criteria of solubility of theacids being included in the context of the present invention.

The organic solvents can be used alone or as mixtures with anothersolvent in accordance with the invention or otherwise. Thus, theseorganic solvents can be used with water.

Advantageously, the organic solvent of the invention has a boiling pointwhich is compatible for carrying out the crystallization underindustrially exploitable operating conditions. Thus, suitable solventsadvantageously have a boiling point of between 40° C. and 250° C.,preferably between 80° C. and 120° C. The crystallization can beperformed at atmospheric pressure or under pressure.

The solvents which are suitable for the invention also advantageouslyhave a certain affinity for water, allowing washing of the crystallineadipic acid with water to remove the traces of organic crystallizationsolvent.

The process of the invention is carried out according to the knowntechniques of crystallization processes. Briefly, this process comprisesa step of dissolution under hot conditions of the carboxylic acid to becrystallized, followed by a step of cooling after optionally filteringthe hot acid solution. It is also possible to concentrate the hot acidsolution before it is cooled. This crystallization can be improved byany known means and in particular by the use of microwaves.

Needless to say, the crystallization process can be repeated. Similarly,the solution recovered after a crystallization can be concentrated whilehot and then cooled in order to recover a further crop of acid.

The present process can be applied to carboxylic acids and moreparticularly to an adipic acid from various synthetic processes, suchas, for example, the adipic acid from the nitric oxidation ofcyclohexanol and/or cyclohexanone, from the double hydroxycarbonylationof butadiene or alternatively from the direct oxidation of cyclohexanein air.

The process of the invention applies particularly to the process for thehydroxycarbonylation of butadiene, which consists of a firsthydroxycarbonylation of butadiene leading to a mixture of pentenoicacids, mainly 3-pentenoic acid, and a second hydroxycarbonylationperformed on the pentenoic acids obtained in the first reaction andleading to adipic acid also containing a certain amount of2-methylglutaric acid and 2-ethylsuccinic acid, as well as othercompounds already originating from the first hydroxycarbonylationreaction, such as γ-valerolactone, unconverted pentenoic acids andmethylbutenoic acid. These organic impurities are advantageously removedby a crystallization process in accordance with the invention.

The process of the invention also applies more particularly to thetreatment and recovery of carboxylic acid such as adipic acidsynthesized by direct oxidation of a hydrocarbon such as cyclohexanewith air. This process is described in particular in patent applicationWO-A-94/07834.

This document describes the oxidation of cyclic hydrocarbons into thecorresponding diacids, in a liquid phase comprising a solvent, using agas containing oxygen and in the presence of an oxidation catalyst suchas a cobalt compound, the said solvent comprising an organic acidcontaining only primary or secondary hydrogen atoms. That patent moreparticularly develops the phases for treating the final reactionmixture. This treatment consists in separating the diacid formed, bycooling the mixture in order to bring about precipitation of the saiddiacid, in separating, by filtration, the diacid from two liquid phases,a nonpolar phase which is recycled, and a polar phase which is also atleast partially recycled after an optional hydrolysis and a separationof an additional amount of diacid.

That patent more particularly provides a solution for the one-stepoxidation of cyclohexane into adipic acid with industrially acceptableselectivity, but it does not provide an industrially applicable solutionto the treatment of the reaction mixture obtained from the oxidation,taking into account the separation of the various products andby-products of the reaction, the unconverted materials and in particularthe recycling of the catalyst.

Patent EP-A-0,772,581 describes a more complete process for treating thereaction mixture obtained from the direct oxidation of cyclohexane intoadipic acid, as well as the recycling of the catalyst. However, in thatprocess, the adipic acid is also crystallized from acetic acid which isa corrosive and expensive solvent, requiring recovery and regenerationoperations which greatly affect the general economy of the adipic acidmanufacturing process.

To overcome these drawbacks, the Applicant has provided, in its patentEP 084,980, a process for separating adipic acid from acetic acid andthen crystallization of this acid from water. However, it has beenobserved that the crystallization in water of adipic acid does notcompletely remove the traces of the organic solvent used for theoxidation reaction, in particular when this solvent is a water-misciblecarboxylic acid such as acetic acid.

This presence of traces of solvent can complicate the secondrecrystallization of the adipic acid, in particular as regards the shapeof the crystals obtained.

In addition, the Applicant has also found that the presence of oxidationintermediates, such as cyclohexanone, cyclohexanol, cyclohexyl esters,hydroxycarboxylic acids and lactones, is liable to interfere with theseparation and purification of the adipic acid. The Applicant hasprovided, in particular in its French patent application 98/02928, whichis not yet published, additional treatment processes for removing theorganic impurities.

The crystallization process provided by the present invention removesmost of these organic impurities without requiring the use of acorrosive crystallization solvent such as acetic acid or additionaltreatments. Thus, the acid crystallized according to the process of theinvention may be purified more readily by a further crystallization fromwater, if necessary, to achieve the high purity specifications requiredin the applications for the manufacture of polyamide or polyurethane.

A subject of the invention is also a process for preparing carboxylicacids by oxidation of a hydrocarbon by direct oxidation and treatment ofthe reaction medium obtained.

A subject of the invention is, more particularly, a process for treatingthe reaction mixture obtained from the direct oxidation of hydrocarbonsinto carboxylic acid, in particular using molecular oxygen or a gascontaining it, in the liquid phase, in a solvent and in the presence ofa catalyst dissolved in the reaction medium, characterized in that thesaid process comprises:

when the composition of the reaction mixture allows it, a separation ofthe phases by settling into two liquid phases: a nonpolar upper phase,essentially containing the unconverted hydrocarbon, and a polar lowerphase essentially comprising the solvent, the acids formed, the catalystand some of the other reaction products and unconverted hydrocarbon;

a distillation of the polar lower phase or, where appropriate, of all ofthe reaction mixture, thus separating, on the one hand, a distillatecomprising at least some of the most volatile compounds such as theunconverted hydrocarbon, the solvent, the reaction intermediates and thewater, and, on the other hand, the distillation residue comprising thecarboxylic acids formed and the catalyst;

the addition of an organic solvent in accordance with the invention, ora mixture comprising at least one organic solvent in accordance with theinvention, to the distillation residue,

the separation of the catalyst, which may have precipitated during theaddition of the organic solvent, by filtration of the medium, forexample,

the crystallization of the carboxylic acid from the organic solution ofthe distillation residue,

optionally, a recrystallization of the carboxylic acid recovered fromthe same solvent or from water.

The separation by settling into two phases of the reaction mixturesubjected to the process of the invention depends essentially on thereaction solvent used, the amount of hydrocarbon converted and the watercontent in the medium.

The hydrocarbons used as starting materials in the process of theinvention are, more particularly, alkanes, cycloalkanes andalkylaromatic hydrocarbons containing from 3 to 20 carbon atoms.

Among these hydrocarbons, cycloalkanes, in particular those which have aring containing from 5 to 12 carbon atoms, are without doubt the mostimportant, since their oxidation leads to dicarboxylic acids.

The most advantageous hydrocarbon is cyclohexane, the oxidation of whichleads to adipic acid, one of the base compounds in polyamide 6-6 and oneof the diacids most commonly used.

For the purposes of simplicity, the invention will be described verygenerally with reference to the treatment of the reaction mixturesobtained from the oxidation of cyclohexane into adipic acid, but theprocess can also be applied to mixtures obtained from the oxidation ofother hydrocarbons, and more particularly of other cycloalkanes.

The cyclohexane phase obtained in the optional step of separation bysettling is usually reintroduced into a cyclohexane oxidation operation.

The solvent used in the oxidation of the hydrocarbon, preferablycyclohexane, is an at least partial solvent for the carboxylic acidwhose preparation is intended. This solvent can be very varied innature, provided that it is not substantially oxidizable under thereaction conditions. It can be chosen in particular from polar proticsolvents and polar aprotic solvents. As polar protic solvents, mentionmay be made, for example, of carboxylic acids containing only primary orsecondary hydrogen atoms, in particular aliphatic acids containing from1 to 9 carbon atoms, perfluoroalkylcarboxylic acids such astrifluoroacetic acid, water and alcohols. As polar aprotic solvents,mention may be made, for example, of lower alkyl esters (alkyl radicalpreferably containing from 1 to 4 carbon atoms) of carboxylic acids, inparticular of aliphatic carboxylic acids containing from 1 to 9 carbonatoms or of perfluoroalkylcarboxylic acids, tetramethylene sulphone (orsulpholane), and aliphatic nitriles such as acetonitrile.

Acetic acid is generally preferred, in particular when the substrate tobe oxidized is cyclohexane.

The catalyst preferably contains cobalt, manganese, a mixture of cobaltwith one or more other metals such as manganese, chromium, iron,zirconium, hafnium or copper, or a mixture of manganese with one or moreother metals such as chromium, iron, zirconium, hafnium or copper. Amongthe cobalt-based mixtures, catalysts comprising either cobalt andchromium, cobalt, chromium and zirconium, cobalt and iron, cobalt andmanganese or cobalt and zirconium and/or hafnium are more particularlysuitable. This catalyst is used for the oxidation of cyclohexane in theform of compounds of these metals which are soluble in the reactionmedium.

The reaction mixture to be treated by the process of the inventioncontains, as a guide, on a weight for weight basis, from 1% to 99% ofunconverted hydrocarbon, from 1% to 40% of carboxylic acids formed, from0.1% to 10% of water, from 0.001% to 5% of the metal(s) contained in thecatalyst, and from 0.1% to 10% of other oxidation reaction products, theremainder consisting of the solvent.

The step of distillation of the lower phase, or where appropriate of thereaction mixture, is carried out such that most, and as far as ispossible all, of the unconverted cyclohexane which may still be presentin this lower phase is separated from the adipic acid. The reactionintermediates, such as cyclohexanol, cyclohexanone, cyclohexyl acetateand lactones (esentially butyrolactone and valerolactone) are alsoseparated out, as is the solvent, preferably comprising a carboxylicacid.

The distillation step is generally carried out at a temperature from 25°C. to 250° C. and at an absolute pressure of between 10 Pa andatmospheric pressure. Preferably, the temperature of the mixture duringthe distillation will be maintained between 70° C. and 150° C.

The distillation can, if necessary, be carried out in several successivesteps, in particular in the preferred mode in which it is desired toremove the majority, for example more than 90% and even more than 99%,of the aliphatic carboxylic acid solvent.

One advantageous variant of the process of the invention consists inintroducing steam into the reaction mixture before or during thedistillation step.

This operation can allow better entrainment of certain compounds presentin the mixture subjected to the distillation. It can also achieve apartial or complete hydrolysis of the carboxylic esters which may alsobe found in the mixture to be distilled.

The distillate obtained in the distillation operation described abovecomprises the various volatile compounds and water. These volatilecompounds are valorizable and thus recycled in a new cyclohexaneoxidation reaction, after an at least partial removal of the water, byany known means, in particular by azeotropic distillation.

In accordance with the invention, an organic solvent or a mixturecomprising at least one organic solvent in accordance with the inventionis added to the distillation residue to dissolve the adipic acid formedand optionally to precipitate the catalysts. The amount of solvent addedrepresents from 0.1 to 20 times the weight of the mixture obtained afterthe said distillation. Preferably, the amount of solvent addedrepresents from 0.5 to 10 times this weight.

This operation thus consists in dissolving the adipic acid in theminimum amount of hot solvent, which optionally allows the catalyst tobe separated out by hot filtration of the solution. The crystallizationitself is performed according to the usual techniques, by gradualcooling of the organic solution. Generally, the solution is seeded usingcrystals of adipic acid.

The adipic acid obtained by this crystallization can then berecrystallized from water, in order to achieve the purity required forthe main applications in which it is used.

Variants can be performed without departing from the context of thepresent invention. Thus, certain operations can be carried out beforethe step of crystallization of the adipic acid from an organic solvent.It is advantageous, for example, to perform an additional oxidation ofthe intermediate oxidation compounds present in the polar liquid phase,after most, and preferably all, of the unconverted cyclohexane has beenseparated out and before the other compounds and the carboxylic acidsolvent have been separated out. This additional oxidation can beperformed with molecular oxygen or a gas containing it, the oxidationbeing catalysed by the initial catalyst still present in the said polarphase, or alternatively using an oxygen donor such as hydrogen peroxideor an organic hydroperoxide.

Such an additional oxidation is then followed by the total or partialseparation of the carboxylic acid solvent, before the step ofcrystallization of the adipic acid in an organic solvent.

After this optional additional oxidation operation, the adipic acid iscrystallized as indicated above.

The process of the invention can also be completed by a reaction toreduce the reducible impurities which may be present in thetechnical-grade adipic acid obtained by crystallization from an organicsolvent.

Such a reduction is advantageously a hydrogenation with hydrogen, afterredissolving the technical-grade adipic acid in a solvent such as water,if necessary in the presence of a hydrogenation catalyst not dissolvedin the medium. The hydrogenation catalyst is separated from the mediumat the end of the reaction.

Finally, the recrystallization from water can also be preceded by anoxidative finishing operation using nitric acid or alternatively by aknown treatment of adsorption and decolourization, for example usingcarbon black.

The process of the invention makes it possible to obtain, in particular,after the recrystallization from water, adipic acid which can satisfythe specifications set for its use in the synthesis of polyamide 6-6 orpolyurethanes.

Other details and advantages of the invention will emerge more clearlyin the light of the examples given below, purely as a guide and for thepurposes of illustration.

EXAMPLE 1

Preparation of Oxidation Masses

A jacketed titanium 1.5 l autoclave equipped with a six-blade turbomixerand various openings for the introduction of the reagents and fluids orfor the removal of the reaction products and fluids, and which has beenpurged beforehand with nitrogen, is loaded at room temperature with thefollowing:

Cobalt acetate tetrahydrate  4.0 g (16 mmol) Acetic acid   357 gCyclohexane 292.5 g Cyclohexanone  3.2 g (32.7 mmol)

After closing the autoclave, the nitrogen pressure is brought to 20 bar,the stirring (1000 rpm) is started and the temperature is brought to105° C. over 20 minutes. The nitrogen is then replaced with 20 bar ofdepleted air (5% oxygen). The flow rate of inlet gas is adjusted to 250liters per hour.

After an induction of about ten minutes, during which there is noconsumption of oxygen, the temperature rises by 2 to 3° C. and theoxygen begins to be consumed. The inlet oxygen titre is gradually raisedto 21%. The oxygen titre at the reactor outlet remains less than 5%throughout the test. The temperature in the autoclave ranges between104.9 and 105.1° C.

When 50 liters of oxygen have been consumed (degree of conversion ofabout 20%), continuous injection of the liquid phase is commenced:injection of an acetic acid solution containing 1.1% by weight of cobaltacetate tetrahydrate and 1.45% by weight of cyclohexanone at a flow rateof 4.6 ml/min (stabilized regime) and injection of cyclohexane at a flowrate of 5 ml/min (stabilized regime). The liquid product is storedcontinuously in a 7 litre decanter at 70° C.

After 380 min from the start of the reaction, the air is graduallyreplaced with nitrogen and the contents of the autoclave are transferredinto the decanter. The decanter contains a two-phase mixture. The upper,essentially cyclohexane phase, which contains only a small amount ofproducts and cobalt, is separated out. The acetic lower phase (2675 g)contains most of the oxidation products and the cobalt.

The acetic phase is subjected to a first distillation under thefollowing conditions:

pressure: 60 kPa

temperature: 135° C.

The distillation residue 1 is subjected to a further, more rigorousdistillation intended to remove the volatile organic compounds itcontains by means of an injection of steam at 150° C. under a pressureof 10 kPa. The results obtained are collated in the table below:

Distillation Distillation Compound Initial untreated ma residu residuCyclohexanone 285 mmol 90 mmol negligible Cyclohexyl acetate 19.0 mmol37.0 mmol negligible Free cyclohexanol 245 mmol 58 mmol negligibleGlutaric acid* 249 mmol 249 mmol 249 mmol Succinic acid* 164.9 mmol164.9 mmol 164.9 mmol Adipic acid* 2115 mmol 2115 mmol 2115 mmolHydroxycaproic acid 50.0 mmol 50.0 mmol 50.0 mmol 3-Hydroxyadipic acid104 mmol 104 mmol 104 mmol Butyrolactone 85.3 mmol 58.0 mmol negligibleValerolactone 34.0 mmol 12.2 mmol negligible Total mass 2675 g 595 g 452g *Total acid (free and esterified)

EXAMPLE 2

Crystallization from Water

A portion of the distillation residue 2 (112 g) is subjected to acrystallization from water (250 g). The mixture is heated to 70° C. andis then gradually cooled to room temperature.

After filtration and washing with water, 60 g of crude adipic acid areobtained.

A recrystallization from water of this crude adipic acid gives apurified adipic acid (A) containing:

succinic acid: 0.0003%

glutaric acid: <0.0001%

cobalt: <0.0002%

The cobalt catalyst is in the crystallization water and the washingwater.

EXAMPLE 3

Crystallization from Acetone

Another portion of the distillation residue 2 of Example 1 (225 g) isdiluted in 700 ml of acetone and brought to reflux. The organicmaterials are dissolved. The undissolved cobalt salts are filtered offwhile hot and recovered for recycling.

The filtrate is gradually cooled to room temperature (about 20° C.).

After filtration and washing with acetone, 120 g of crude (ortechnical-grade) adipic acid are obtained.

A recrystallization from water of this crude adipic acid gives apurified adipic acid (B) containing:

succinic acid: 0.0002%

glutaric acid: <0.0001%

cobalt: <0.0002%

The adipic acid batches (A) and (B) are subjected to a heating test.This test consists in heating 50 g of each batch to 215° C. for 205 minand then placing each of them in 415 ml of aqueous 5% ammonia solution.The absorbence at 454 nm of the ammonium adipate solutions obtained isthen measured.

The results below are obtained, expressed as relative absorbences, thereference adipic acid (A) representing the value 1:

adipic acid (A): 1

adipic acid (B): 0.25

The purified adipic acid (B) according to the present invention containsfewer impurities liable to become coloured on heating.

EXAMPLE 4

Crystallization from 1,4-dioxane

A portion of the distillation residue 1 (112 g) of Example 1 issubjected to a crystallization from 1,4-dioxane (250 g). The mixture isheated to 70° C., filtered while hot and then gradually cooled to roomtemperature.

After filtration and washing with water, 50 g of crude adipic acid areobtained.

A recrystallization from water of this crude adipic acid gives apurified adipic acid (C) containing:

succinic acid: 0.0002%

glutaric acid: <0.0001%

cobalt: <0.0002%

The cobalt catalyst was recovered by hot filtration of the dioxanesolution.

EXAMPLE 5

Crystallization from Acetonitrile

A portion of the distillation residue 1 (112 g) of Example 1 issubjected to a crystallization from acetonitrile (450 g). The mixture isheated to 80° C., filtered while hot and then gradually cooled to roomtemperature.

After filtration and washing with water, 65 g of crude adipic acid areobtained.

A recrystallization from water of this crude adipic acid gives apurified adipic acid (D) containing:

succinic acid: 0.0005%

glutaric acid: <0.0001%

cobalt: <0.0002%

The cobalt catalyst was recovered by hot filtration of the acetonitrilesolution.

EXAMPLE 6

Crystallization from Isopropanol

A portion of the distillation residue 1 (112 g) of Example 1 issubjected to a crystallization from isopropanol (250 g). The mixture isheated to 100° C., filtered while hot and then gradually cooled to roomtemperature.

A portion of the distillation residue 1 (112 g) of Example 1 issubjected to a crystallization from butyl acetate (250 g). The mixtureis heated to 100° C., filtered while hot and then gradually cooled toroom temperature.

After filtration and washing with water, 58 g of crude adipic acid areobtained.

A recrystallization from water of this crude adipic acid gives apurified adipic acid (E) containing:

succinic acid: 0.0002%

glutaric acid : <0.0001%

cobalt: <0.0002%

The adipic acid batches (A) and (C) to (E) are subjected to a heatingtest.

This test consists in heating 50 g of each batch to 215° C. for 205 minand then placing each of them in 415 ml of aqueous 5% ammonia solution.

The absorbance at 454 nm of the ammonium adipate solutions obtained isthen measured.

The following results are obtained, expressed as relative absorbances,the reference adipic acid (A) representing the value 1:

Purified Relative absorbance adipic acids at 454 nM A (Ex. 3) 1 C (Ex.4) 0.3 D (Ex. 5) 0.2 E (Ex. 6) 0.5

The adipic acids (B), (C) and (D) purified according to the presentinvention contain fewer impurities liable to become coloured on heatingthan the adipic acid (A), thus demonstrating the effects of thecrystallization from an organic solvent.

In another embodiment of the process of the invention, thecrystallization from an organic solvent in accordance with the inventioncan be carried out on adipic acid crystallized from water. Thiscrystallization from an organic solvent can be completed again bypurification treatments as described above and/or by anothercrystallization from water.

The examples given above can be applied to a reaction medium obtainedfrom the hydroxycarbonylation of butadiene.

What is claimed is:
 1. A process for treating the reaction mixtureobtained from the direct oxidation of hydrocarbons into carboxylicacids, in the liquid phase, in a solvent and in the presence of acatalyst dissolved in the reaction medium, said process comprising:separating when the composition of the reaction mixture allows it, thephases by settling into two liquid phases: a nonpolar upper phase,comprising the unconverted hydrocarbon, and polar lower phase comprisingthe solvent, the acids formed, the catalyst and some of the otherreaction products and the unconverted hydrocarbon; distilling the polarlower phase or, where appropriate, all of the reaction mixture, thusseparating, on the one hand, a distillate comprising at least some ofthe most volatile compounds, the solvent, the reaction intermediates andsubstantially all of the water, and, on the other hand, the distillationresidue comprising the carboxylic acids formed and the catalyst; addingto the distillation residue an organic solvent in which the carboxylicacid has a solubility of less than or equal to 15% by weight at 20° C.or a mixture comprising at least one of said organic solvent, saidorganic solvent being selected from the group consisting of ethers,alcohols, esters, nitriles, amides, sulphoxides, carbonates, andhalogen, nitro or phosphorous-containing compounds; optionally filteringthe medium obtained; and recovering said carboxylic acids bycrystallization of the acids formed by cooling and/or evaporation of thesaid organic solvent.
 2. The process according to claim 1, wherein thesolubility of the carboxylic acid in the solvent at a temperature of100° C., or at its boiling point if this is lower than 100° C., is atleast 5% higher than that at 20° C. in the same solvent.
 3. The processaccording to claim 1, wherein the hydrocarbon used as starting materialis selected from these group consisting of alkanes, cycloalkanes andalkylaromatic hydrocarbons having from 3 to 20 carbon atoms.
 4. Theprocess according to claim 3, wherein the hydrocarbon used as startingmaterial comprises cycloalkanes which contain a ring having from 5 to 12carbon atoms.
 5. The process according to claim 4, wherein thehydrocarbon used as the starting material is cyclohexane.
 6. The processaccording to claim 5, wherein the carboxylic acid formed is adipic acid.7. The process according to claim 1, wherein the crystallizationsolvents are selected from the group consisting of 1,4-dioxane; diglyme(diethylene glycol dimethyl ether); tetrahydrofuran; aliphatic,cycloaliphatic, aromatic or arylaliphatic ketones such as acetone,methyl isobutyl ketone, methyl ethyl ketone, methyl isopropyl ketone,methyl phenyl ketone, cyclohexanone, n-butanol; isopropanol;3-methoxyethanol; acetonitrile; dimethylformamide; acetamide;dichloromethane; ethyl acetate; 1,2-dichloroethane; dimethyl sulphoxide;nitromethane; and N-methylpyrrolidone.
 8. The process according to claim1, wherein the solvent used in the step of oxidation of the hydrocarbonis at least a partial solvent for adipic acid and is selected from thegroup consisting of polar protic solvents and polar aprotic solvents. 9.The process according to claim 8, wherein the solvent used in theoxidation step comprises aliphatic acids having from 1 to 9 carbonatoms.
 10. The process according to claim 1, wherein the catalystcomprises cobalt, manganese, a mixture of cobalt with one or more othermetals chosen from manganese, chromium, iron, zirconium, hafnium andcopper or a mixture of manganese with one or more other metals selectedfrom the group consisting of chromium, iron, zirconium, hafnium andcopper.
 11. The process according to claim 10, wherein the catalystcomprising cobalt-based mixtures comprising either cobalt and chromium,or cobalt, chromium and zirconium, or cobalt and iron, or cobalt andmanganese or cobalt and zirconium and/or hafnium.
 12. The processaccording to claim 1, wherein the step of distillation of the polarlower phase, or where appropriate of the reaction mixture, is carriedout at a temperature from 25° C. to 250° C., and under an absolutepressure of between 10 Pa and atmospheric pressure.
 13. The processaccording to claim 1, wherein the carboxylic acid obtained aftercrystallization from the organic solvent is recrystallized from water.14. Process according to claim 13, characterized in that the hydrocarbonused as starting material is cyclohexane.
 15. Process according to oneof claims 8 to 14, characterized in that the solvent used in theoxidation of the hydrocarbon is an at least partial solvent for adipicacid and is chosen from polar protic solvents and polar aproticsolvents.
 16. Process according to one claims 8 to 15, characterized inthat the solvent is chosen from aliphatic acids containing from 1 to 9carbon atoms and is preferably acetic acid.
 17. Process according to oneof claims 8 to 16, characterized in that the catalyst contains cobalt,manganese, a mixture of cobalt with one or more other metals chosen frommanganese, chromium, iron, zirconium, hafnium and copper or a mixture ofmanganese with one or more other metals chosen from chromium, iron,zirconium, hafnium and copper.
 18. Process according to claim 17,characterized in that the catalyst is chosen from cobalt-based mixturescomprising either cobalt and chromium, cobalt, chromium and zirconium,cobalt and iron, cobalt and manganese or cobalt and zirconium and/orhafnium.
 19. Process according to one of claims 8 to 18, characterizedin that the step of distillation of the polar lower phase, or whereappropriate of the reaction mixture, is carried out at a temperaturefrom 25° C. to 250° C., preferably from 70° C. to 150° C., and under anabsolute pressure of between 10 Pa and atmospheric pressure.
 20. Processaccording to one of claims 8 to 19, characterized in that the adipicacid obtained after crystallization from the organic solvent isrecrystallized from water.